1. Field of the Invention
The present invention relates to a liquid ejecting apparatus having a liquid ejecting head for ejecting liquid drops from a nozzle opening.
2. Description of the Related Art
As a typical example of a conventional liquid ejecting apparatus, there is an ink jet recording apparatus having an ink jet recording head for recording images. As other liquid ejecting apparatuses, for example, an apparatus having a coloring material ejecting head used to manufacture a color filter for a liquid crystal display, an apparatus having an electrode material (conductive paste) ejecting head used to form an electrode for an organic EL display and a face emission display (FED), an apparatus having a biological organic substance ejecting head used to manufacture biological chips, and an apparatus having a sample ejecting head as a precise pipette may be cited.
The ink jet recording apparatus which is a typical example of the liquid ejecting apparatus makes comparatively little noise during printing and can form small dots in high density, so that it is used recently in many printings including color printing.
Such an ink jet recording apparatus generally has an ink jet recording head (liquid ejecting head) which is loaded on a carriage and moves back and forth in the width direction (head scanning direction) of a recording medium such as a recording paper and a medium feed means for moving the recording medium in the perpendicular direction (medium feed direction) to the head scanning direction.
In the ink jet recording apparatus, printing is carried out by ejecting ink drops (liquid drops) to a recording medium from a recording head in correspondence to print data. And, the recording head loaded on the carriage is enabled to eject ink of various colors such as black, yellow, cyan, and magenta, thus not only text printing by black ink but also full-color printing are enabled by changing the ejection rate of various colors of ink.
The ink jet recording apparatus, to charge ink in the ink flow path in the recording head at the start time of use or to prevent the nozzle opening from clogging due to volatilization of an ink solvent, has an ink suction function for forcibly suck in and discharging ink from the nozzle opening of the recording head. The forcible ink discharging process executed to eliminate clogging of the recording head or discharge residual bubbles in the recording head is called a cleaning operation. And, the cleaning operation is executed in a case of restart of printing after suspension of the recording apparatus for many hours or when a user recognizes print quality faults such as print blurs and operates the cleaning switch.
The cleaning operation seals the nozzle forming face of the recording head by the capping means, applies a negative pressure into the capping means, thereby discharges ink from the nozzle openings of the recording head, sucks in ink discharged in the capping means, and sends it to the waste ink tank. Thereafter, a sequence of wiping the nozzle forming face of the nozzle plate of the recording head is executed by the wiping means composed of an elastic plate such as rubber.
As a means for applying a negative pressure into the capping means, the so-called tube pump in which the structure is comparatively simple and can be miniaturized and the machinery sucking in and discharging ink is free of contamination is generally used. The tube pump, as shown in FIG. 15, has a flexible tube 50 in which a part thereof is curved in a circular ring shape and the outer periphery thereof is supported by a pump frame (not shown in the drawing) and a roller member 51 for rotating the inner periphery of the circular ring-shaped part of the flexible tube 50 using the power of the paper feed motor.
And, in the tube pump, the roller member 51 rotates by sequentially crushing the circular ring-shaped part of the flexible tube 50, thus pressure is generated in the flexible tube 50 to apply a negative pressure to the capping means. As mentioned above, the tube pump forcibly discharges ink from the recording head by a negative pressure and moreover, sucks ink discharged in the capping means and sends it to the waste ink tank.
Further, for the structure of the tube pump, in place of the structure that the tubes 50 curved in a circular ring shape are pulled out and crossed in the opposite directions as shown in FIG. 15, a constitution that both ends of the flexible tubes 50 curved in a circular ring shape are pulled out in the same direction and are bundled in the same plane as shown in FIG. 16 is proposed. This constitution has no tube crossing part unlike the tube pump shown in FIG. 15, so that the whole tube pump becomes thin and for example, even when two tubes 50 are installed in parallel to increase the pump capacity, the thickness thereof is suppressed to two times of the tube diameter.
In the conventional ink jet recording apparatus mentioned above, when starting the tube pump in the stop state, without controlling the start position of the roller member 51, the roller member 51 can start rotation from an optional position.
However, in the tube pump shown in FIG. 15 or 16, the suction amount structurally varies in accordance with the start position of the roller member 51. Therefore, particularly when the set value of the suction amount (rotation amount) is small, a problem arises that when the start position of the roller member 51 is changed, an actual suction amount varies.
Precisely speaking, the problem is that in the tube shown in FIG. 15 or 16, structurally, there is a position where the flexible tube 50 cannot be crushed by the roller member 51, that is, a leak point and when the roller member 51 is stopped at the leak point, a leak of liquid may be generated in the tube pump. Concretely, in the tube pump shown in FIG. 15, the part X where the flexible tubes 50 are crossed is a leak point and in the tube pump shown in FIG. 16, the part X where the flexible tubes 50 are bundled is a leak point.
And, at the start time of the suction operation, as shown in FIG. 17A, when the roller member 51 starts to move from a position far away from the leak point X, the distance from the movement start position to the leak point X is long, so that the suction amount increases accordingly. On the other hand, as shown in FIG. 17B, when the roller member 51 starts to move from a position close to the leak point X, at the point of time when a negative pressure is generated slightly after rotation start, the roller member 51 reaches the leak point X, and the magnitude of negative pressure is reduced due to a leak there, and the suction amount is reduced accordingly.
FIG. 17C is a graph showing the relationship between the rotation time [second] of the pump and the magnitude of negative pressure [−Pa], and a symbol A in the drawing indicates a negative pressure curve when the suction operation is started from the state shown in FIG. 17A, and a symbol B indicates a negative pressure curve when the suction operation is started from the state shown in FIG. 17B. Furthermore, in the graph shown in FIG. 17C, a negative pressure curve of a pump having no leak point is shown for comparison.
As shown in FIG. 17C, in both cases A and B, at the point of time when the roller member 51 reaches the leak point X, the magnitude of negative pressure is reduced, so that the suction amount is deduced in comparison to the pump having no leak point. And, the degree of reduction in the suction amount accompanying the leak at the leak point X is larger in the case B (FIG. 17B) than in the case A (FIG. 17A).
As mentioned above, in the tube pump having the leak point X, when the movement start position of the roller member 51 is changed, an actual suction amount is varied, and when the set value of the suction amount is small, variations of about ±30% are caused, and when the set value of the suction amount is medium, variations of about ±10% are caused. Further, when the set value of the suction amount is large, variations of about ±5% are caused, and this level of variations may be considered to be within the tolerance, so that in a case of suction of a large mount, the movement start position of the roller member 51 provides no trouble.
Further, if the roller member 51 is stopped at the position of the leak point X when the tube pump is stopped at the end time of the suction operation, ink already sucked by the tube pump flows backward on the capping means side in the negative pressure state. When a backflow of sucked ink is generated, the negative pressure in the capping means is not released normally, causing print faults such as color mixture and non-ejection.
Furthermore, among the conventional pump tubes, there is a type that a pair of roller members are pressed against a flexible tube curved in a U shape. In this kind of pump tube, in correspondence with the rotational motion of the roller members, a state that only one of the pair of roller members presses the tube and a state that both roller members press the tube are generated. And, in a state that both roller members are pressed against the tube, compared with a state that one roller member is pressed against the tube, the load for the motor which is a driving source for the tube pump is doubled.
And, when the pressing state by both roller members is generated immediately after rotation start of the roller members, before the rotational speed of the roller members reaches a preset value, a high load is applied to the motor of the pump. When the rotational speed of the roller members is low, the inertia force of the roller members is also small. As a result, when a high load is applied to the motor of the pump, the motor may step out.
The step-out of the motor may occur also at the position of the leak point X of the tube pump having a circular ring-shaped tube shown in FIGS. 17A and 17B.